1. Field of the Invention
This invention generally relates to an array antenna system for directing and receiving electromagnetic energy and particularly to a fiber optic true time-delay array antenna feed system.
2. Description of the Related Art
An antenna array is a stationary group of individual radiating (and collecting) elements that collectively form electronically steerable transmit (or receive) beams. The electromagnetic radiation (EMR) signal driving (and received by) each element is identical to (from) each other element except for the relative time delays present. Despite their wide use in various applications, phased array antenna systems suffer from several drawbacks. First, routing numerous conventional coaxial or waveguide transmission lines is unwieldy, heavy, expensive and lossy. These implementation difficulties have led to various different optical schemes for phase control, which offer small size and weight, large bandwidths, low loss, low crosstalk and immunity from electromagnetic interference (EMI). Second, phased array antenna system performance deteriorates as the signal bandwidth increases. For example, wide bandwidth operation (e.g., narrow pulse or frequency swept) results in antenna gain reduction and in undesirable beam distortions (xe2x80x9cbeam squintingxe2x80x9d) characterized by beam steering with frequency.
Time-delay modulators are required to allow wide instantaneous bandwidth operation but the difficulty in implementing time-delay modulators has precluded array realization, especially for large aperture, numerous-element arrays now desired for high resolution and high antenna gain. Instead, current phased array antennas comprise antenna elements having identical radio frequency (RF) signals except for relative phase, which is limited to 0 to 2xcfx80 radians. That is, relative time delays are limited to one period of the radio frequency being used.
Nevertheless, some time-delay techniques have been proposed. The technique of electromechanical optical fiber stretching is shown in U.S. Pat. No. 4,814,774, Herczfeld (1989). Here time delays are introduced into the signal delivered to each antenna element by stretching the fiber-optic link feeding each element. However, to produce significant time delays (xcx9c1 ns) while avoiding fiber breakage, long fiber lengths are required and so large stretching forces (piezo-electric cylinders) are required. Large piezos are fragile, require high voltages and have limited speed of response.
In Toughlian et al., A Photonic Variable RF Delay Line for Phased Array Antennas, IEEE J. Lightwave Tech., LT-8, pp. 1824-1828, 1990, Toughlian and Zmuda utilize integrated-optic frequency shifting and diffraction gratings to generate an optical modulated carrier that is steered and delayed according to the radio frequency. This technique relies on complex tight-tolerance bulk optics that tend to be less rugged. Also, the true-time-delay performance has limited bandwidth.
An acousto-optic diffraction scheme proposed and demonstrated in the report Stilwell et al., Fiber Optic Feed, Naval Research Laboratory Memorandum Report 6741, Nov. 6, 1990, uses optically-sampled acousto-optic diffracted interference beams to generate multiple phase-correlated signals to drive antenna elements. However, this optical heterodyne system tends to have narrow instantaneous bandwidth and relies on tight-tolerance bulk optics.
Various different switching schemes involving switching in and out or selecting various different discrete fiber-optic delays have been proposed. Ng et al., The First Demonstration of an Optically Steered Microwave Phased Array Antenna Using True Time-Delay, IEEE J. of Lightwave Tech., 9, pp. 1124-1131, 1991 and U.S. Pat. No. 5,051,754, Newberg (1989) have taught true time-delay array steering by switching on and off laser diode driving various different lengths of fiber all combined together to a single photodetector (feeding an array element). Also, integrated-optic spatial light modulators and fiber-optic cross-bar switches have been suggested for use with an array of various lengths of optical waveguides. All of these approaches have limited utility because they give only discrete time delays, are component and control signal intensive (still requiring electronic phase shifters), and have large losses associated with large fanout needed to approximate a continuously variable time delay.
A wavelength dependent, tunable, optical delay system for selectively varying the wavelength of optical light is described in U.S. Pat. No. 4,671,604, Soref. Utilizing a plurality of wavelength filter combinations or cleaved-coupled-cavity (C3) lasers, the wavelength of the optical signal entering the fiber can be varied over a preselected wavelength band by selectively varying the wavelength of the optical signal. However, to effectively and accurately delay the optical signal, the output of all lasers and the wavelength of all tunable filters must be changed equally and simultaneously. The device in Soref is, thus, complicated to operate and has many expensive components. Further, Soref specifies direct RF modulation of the optical source, which results in an interaction between the RF signal and the wavelength control signal.
The object of this invention is to provide a device for dynamically generating a plurality of identical signals with correlated, continuously variable, relative true time-delays for array antennas operating in the radio-frequency, microwave and millimeter wavebands.
A further object of this invention is to provide an apparatus having wideband characteristics for generating true physical time-delay that produces a frequency-dependent phase shift.
Another object is to provide an apparatus in which the control is accomplished by means of a low voltage signal (0-10 volts) signal which tunes the wavelength of the laser source.
Another object is to provide an apparatus suitable for feeding a one- or two-dimensional array in a transmit and/or receive operational mode.
Another object is to provide an apparatus capable of forming a plurality of independently steered beams each containing different electrical signals directed towards different operational objectives.
Another object is to provide an apparatus having no moving parts and does not use optically coherent techniques that require optical beam alignments having strict tolerances to the dimensions of the optical wave.
Another object of this invention is to provide optical antenna feed signals that avoid interaction between the signal to be transmitted (or received) and the delay control signal (i.e., the wavelength of the optical source).
Another object of this invention is to provide array antenna feed signals that completely eliminate the need for phase shifters and complex control signal distribution systems.
The fiber-optic true time-delay array antenna feed is a device for generating a plurality of identical electromagnetic radiation (EMR) signals with continuously variable and correlated time-delay separations. The EMR signal to be transmitted modulates a continuous-wave optical signal from a continuously-tunable narrow-linewidth (laser) source, which is applied to a plurality of high dispersion single-mode optical fibers acting as synchronized true time-delay modulators forming an antenna feed system. Each fiber is chosen to have a different net dispersion. Photodetectors convert the optical intensity back into an EMR electrical signal feeding each of the elements or subarrays of the array antenna.